I am trying to solve three ODEs, two of which are second order. This error occurs when the third equation reaches a state that eliminates the second order terms from the first two equations. Is there a way around this error? The system is still well-behaved, it's just a lower order after this point. The equations become rather long, so rather than including them here, I have attached my notebook file.

Problem background: I am modeling a simple double pendulum problem, where the mass of the second link is unknown and I use an adaptive feedback controller to determine its mass while applying torques to the joints to reach a desired joint trajectory.

Also, I noticed there is a product called Control Systems Professional (Student 2.0.1). Could anyone tell me from experience if this would help here?